In general, a hybrid electric vehicle (HEV) uses two power sources, an internal combustion engine and electric motor. This is the feature of the HEV that differentiates it from a conventional vehicle using only an internal combustion engine. An appropriate powertrain system and shift control technique must therefore be provided for the HEV, so that power transmission of the two power sources may be smoothly maintained.
In conventional schemes for powertrain systems for such HEVs, power from the engine and power from the electric motor are not perfectly separated. Therefore, an optimum electric drive mode is very difficult to realize. In addition, inertial and/or frictional loads of the engine are active under regenerative braking, which may deteriorate efficiency of the regenerative braking.
In particular, because a clutch in a conventional HEV receives both the power of the engine and the power of the electric motor, the clutch is required to have a large torque capacity. Therefore, a hydraulic pump for controlling engagement of the clutch also must have a large capacity in order to ensure sufficient torque for engaging the clutch at a low speed (i.e., at a low shift-speed), which decreases overall efficiency of power of the vehicle.
Considering that a continuously variable transmission (CVT) is usually limited in its torque capacity, it is generally not appropriate for use as a transmission in an HEV, especially for a vehicle larger than a middle-sized sedan that has a relatively powerful engine. When a CVT is used for such a larger sedan, the CVT may experience slippage because of excessive torque.
As an alternative, an automated shift gearbox (ASG) has been proposed as a transmission for an HEV. In this case, an appropriate scheme of the powertrain and algorithm for controlling shift operation thereof must be newly designed in order to obviate shift-shock and/or a period of no power transmission.
FIG. 7 is a graph illustrating a shift-shock of an HEV having an ASG transmission that is conventionally controlled. As shown therein, a starting point of a shift operation, power from all power sources is separated from drive wheels, which causes output torque of the vehicle to abruptly become zero (0). In addition, a transient shift-shock may occur at a point of engagement of synchronizers, i.e., at a point of engagement of a shift-speed, (refer to circle 1), and also at a point of an engagement of the clutch. Since such a shift-shock is caused by a schematic structure of the powertrain system, the structure of a powertrain system and/or an algorithm for its shift operation should be newly designed to eliminate such a shift-shock.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art in this country.